Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H13: New Theoretical Proposals for Topological SuperconductivityFocus

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Sponsoring Units: DMP Chair: Fan Zhang, University of Texas at Dallas Room: LACC 304A 
Tuesday, March 6, 2018 2:30PM  3:06PM 
H13.00001: Novel Superconducting Phases in Monolayer Transition Metal Dichacolgenides Invited Speaker: Kam Tuen Law It was recently discovered that transition metal dichacolgenides (TMDs), such as NbSe2 and MoS2, can be superconducting down to monolayer thickness. Importantly, due to the breaking of inplane mirror symmetry and strong orbital spinorbit coupling (SOC), electrons in monolayer TMDs experience Ising SOC which pins electron spins to the outofplane directions. It has been shown experimentally and theoretically that Ising SOC can strongly enhance the inplane upper critical field of superconducting TMDs several times beyond the Pauli limit [13]. In this talk, we will further explore how Ising SOC can induce novel superconducting phases in monolayer TMDs including a spintriplet nodal superconducting phase and a spinsinglet topological nodal superconducting phase. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H13.00002: Topological LarkinOvchinnikov phase and Majorana zero mode chain in bilayer superconducting topological insulator films LunHui Hu, ChaoXing Liu, FuChun Zhang We theoretically study bilayer superconducting topological insulator film, in which superconductivity exists for both top and bottom surface states. We show that an inplane magnetic field can drive the system into LarkinOvchinnikov (LO) phase, where electrons are paired with finite momenta. The LO phase is topologically nontrivial and characterized by a Z2 topological invariant, leading to a Majorana zero mode chain along the edge perpendicular to inplane magnetic fields. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H13.00003: Topological superconductivity from ferroelectric fluctuations Minseong Lee, HyunJae LEE, Suk Bum Chung, Jun Hee Lee A topological superconductor features at its boundary Majorana fermions, which are potentially applicable for topological quantum computations. Research on the topological superconductivity has been hampered by the scarcity of the real systems. Therefore, there is a strong demand for such an unconventional superconductivity. In this research, we study a heterostructure consisting of a twodimensional electron gas (2DEG) from 5d transition metals sandwiched by ferroelectric materials using firstprinciple DFT calculations. This system possesses suitable conditions for the topological superconductivity: it has a strong spinorbit coupling, and with oddparity phonon modes at low energy due to ferroelectric fluctuations. The band structure calculations show the Rashba type spin splitting, which results from the spinorbit coupling and inversion symmetry breaking. Setting the substrate lattice constant at an appropriate range softens the oddparity phonon mode to induce the strong electronphonon coupling. Our results indicate that appropriately strained substrates with an extremely soft phonon mode may give rise to the topological superconductivity on 2DEG. We will suggest several candidate substrates and experimental probing techniques. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H13.00004: Monopole Superconductivity with Halfinteger Partialwave Symmetries Yi Li The pairing symmetries of superconducting gap functions are usually of integer partialwaves (s,p,d, and etc). In this work, we study a new class of topological superconducting states whose gap functions possess halfinteger partialwave symmetries. These exotic states can exist in systems with topological Fermi surfaces possessing nonzero Chern numbers. When pairing occurs between Fermi surfaces whose Chern numbers are differed by an odd integer, the corresponding superconducting gap function is represented by monopole harmonics with halfinteger monopole charges, and thus carries spinor partialwave symmetries. Topological properties of this class of states, including the surface Majorana modes are studied. We also discuss the possible experimental realizations in Weyl semimetal system and ultracold atom systems. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H13.00005: PseudoLandau Levels of Bogoliubov Quasiparticles in Strained Nodal Superconductors Geremia Massarelli, Gideon Wachtel, John Wei, Arun Paramekanti Motivated by theory and experiments on straininduced pseudoLandau levels of Dirac fermions in graphene and topological materials, we consider its extension for Bogoliubov quasiparticles in a nodal superconductor (SC). We show, using an effective low energy description and numerical lattice calculations for a dwave SC, that a spatial variation of the electronic hopping amplitude or a spatially varying swave pairing component can act as a pseudomagnetic field for the Bogoliubov quasiparticles, leading to the formation of pseudoLandau levels. We propose realizations of this phenomenon in the cuprate SCs, via strain engineering in films or nanowires, or swave proximity coupling in the vicinity of a nematic instability, and discuss its signatures in tunneling experiments. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H13.00006: The 10 phases of spin chains with two Ising symmetries Batel Friedman, Atanu Rajak, Angelo Russomanno, Emanuele Dalla Torre We explore the topological properties of quantum spin1/2 chains with two Ising symmetries. This class of models does not possess any of the symmetries that are required to protect the Haldane phase. Nevertheless, we show that there are 4 symmetryprotected topological phases, in addition to 6 phases that spontaneously break one or both Ising symmetries. By mapping the model to onedimensional interacting fermions with particlehole and timereversal symmetry, we obtain integrable parent Hamiltonians for the conventional and topological phases of the spin model. We use these Hamiltonians to characterize the physical properties of all 10 phases, identify their local and nonlocal order parameters, and understand the effects of weak perturbations that respect the Ising symmetries. Our study provides the first explicit example of a class of spin chains with several topologically nontrivial phases, and binds together the topological classifications of interacting bosons and fermions. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H13.00007: From onedimensional charge conserving superconductors to the gapless Haldane phase Anna Keselman, Patrick Azaria, Erez Berg We discuss the existence of topological superconducting phases in onedimensional, charge conserving systems. We develop a general approach to treat onedimensional systems with multiple flavors and attractive pairing interactions. For a single flavor, i.e. a spinless system, we recover the result that a distinct topological phase does not exist due to absence of a gap to single particles in the bulk. For N flavors, we study the case of SO(N) symmetric interactions, and show that for N > 1, a distinct topological phase, with protected, exponentially localized endmodes, can exist. We discuss in detail the case of N = 3, and show that such a system can realize a gapless analogue of the Haldane phase in spin1 chains. In this phase, although the bulk is gapless to single particle excitations, the ends host spin1/2 degrees of freedom which are exponentially localized and protected by the spin gap in the bulk. We introduce a lattice model that realizes this phase and analyze it both analytically, using Bosonization and RG, and numerically, using DMRG. 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H13.00008: Nonadiabatic Berry phase description of Floquet topological states Ian Mondragon, Aris Alexandradinata, Ivar Martin, Meng Cheng We study Floquet topological states using nonadiabatic Berry phases. We show that these geometric quantities correspond to the centers of Wannier states in the frequency domain, in complete analogy with how conventional Berry phases in equilibrium systems correspond to Wannier centers in position space. This link reveals a natural way to understand the topology of periodically driven phases in terms of the behavior of Wannier centers in the frequency domain. We illustrate the versatility of this approach with a model of a driven Majorana wire. We show that the bulk topology is characterized by a Chern number computed using a nonadiabatic version of the Berry curvature. Furthermore, we find that the corresponding Majorana modes at the edge carry a quantized nonadiabatic polarization in the frequency domain. We discuss higherdimensional systems, such as Floquet Chern insulators, as well as systems with interactions and disorder. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H13.00009: Topological Superconductivity with Spin Valves Tong Zhou, Alex Matos Abiague, Jong E Han, Igor Zutic 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H13.00010: Abstract Withdrawn Magnetic impurities inserted in an swave superconductor give rise to spinpolarized ingap states called YuShibaRusinov (YSR) states. When a set of such impurities are put in proximity with one another, forming for example a chain, they hybridize and they rise to a Shiba band that is described by a pwave, or topological superconductor [1]. In this work, we address address the physical manifestations associated with the dynamics of such impurities. For the single Shiba state, we show that the impurity spin precession pertains to dc charge and spin currents flowing through a normal STM tip tunnel coupled to the superconductor in the vicinity of the impurity [2]. In the case of a Shiba band, we show that such a dynamical setup gives rise to a Hamiltonian that shows nontrivial topological properties, and present the corresponding topological phase diagram. Moreover, we describe how to implement our proposal in an onchip setup, by driving the magnetic impurities or texture by means of the spintransfer torque [3]. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H13.00011: Geometric percolation at the surface of a topological superconductor Sayed Ali Akbar Ghorashi, Yunxiang Liao, Matthew S. Foster We present evidence that strongly suggests the equivalence between disordered surface states of topological superconductors (TSCs) and geometric percolation. Percolation is known to play a role in quantum Hall systems with magnetic fields. Our unexpected result implies that percolation applies to TSC surface states, in the absence of timereversal symmetry breaking. Moreover, the usual "evenodd" effect that occurs in such a system (as identified by Pruisken in the integer quantum Hall effect and by Haldane for spin chains) is found to be absent. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H13.00012: Commutingprojector Hamiltonians for Chiral Topological Phases Built from Parafermions Jun Ho Son, Jason Alicea

Tuesday, March 6, 2018 5:18PM  5:30PM 
H13.00013: Supersymmetry in Closed Chains of Coupled Majorana Modes Zhao Huang, Shinji Shimasaki, Muneto Nitta We consider a closed chain of even number of Majorana zero modes with nearestneighbour couplings which are different site by site generically, thus no any crystal symmetry. Instead, we demonstrate the possibility of an emergent supersymmetry (SUSY), which is accompanied by gapless Fermionic excitations. In particular, the condition can be easily satisfied by tuning only one coupling, regardless of how many other couplings are there. Such a system can be realized by four Majorana modes on two parallel Majorana nanowires with their ends connected by Josephson junctions and bodies connected by an external superconducting ring. By tuning the Josephson couplings with a magnetic flux Φ through the ring,we get the gapless excitations at Φ_SUSY =±fΦ_0 with Φ_0 = hc/2e, which is signaled by a zerobias conductance peak in tunneling conductance. We find this f generally a fractional number and oscillating with increasing Zeeman fields that parallel to the nanowires, which provide a unique experimental signature for the existence of Majorana modes. 
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